The rapid growth of Internet data traffic has driven current fiber optic networks to a new stage where much broader bandwidth and higher capacity are required. Dense wavelength division multiplexed (DWDM) systems with narrow channel spacing and low crosstalk have proven to be a promising solution. Generally in a DWDM system, each channel is represented by a fixed wavelength from a wavelength-fixed laser source and all the different channels are sent into the same optical fiber and transmitted to a receiver end. In order to fully implement a DWDM system with thousands of channels under this wavelength-fixed scheme, service providers in the telecom industry face huge inventory, complexity, and cost problems because of the large number of laser sources and accessories needed, as well as the need for backup lasers and spare parts. However, the use of tunable laser sources in which the lasing wavelength can be tuned over a certain range, for instance, the wavelength band of erbium doped optical-fiber amplifier (EDFA), could dramatically simplify a DWDM system, enable highly flexible and effective utilization of the optical fiber bandwidth, and, thus, can significantly reduce cost to service providers.
Several tunable laser technologies have been investigated over the last decade including distributed feedback Bragg (DFB) grating lasers, distributed Bragg reflector (DBR) laser, vertical-cavity surface-emitting lasers (VCSELs), and external cavity lasers (ECLs). Tunable DFB lasers are in general realized by changing the refractive index of the internal grating either thermally or electrically by which the operating wavelength can be tuned. Although DFB lasers are well behaved and very reliable, they have the disadvantages of low output power and very limited wavelength tuning range (i.e., a range of about 5.0 nm).
DBR lasers have similar structures to DFB lasers but have a grating section separated from an active section. By injecting current into the grating region to change the refractive index, the effective length of the laser cavity is changed and therefore the lasing wavelength. DBR lasers have some advantages such as fast tuning speed, relatively large tuning range (about 40 nm), but suffer drawbacks of wavelength instability, broad linewidth, and large device size.
VCSELs have a gain layer sandwiched by two DBR mirrors. The light is emitted from the top surface of the mirror instead of the edge as in the conventional edge-emitting lasers. This gives VCSELs the biggest advantage in that the laser output can be coupled to a fiber very easily and cost-effectively. The wavelength tuning of VCSELs is realized by injecting current to a micro-electromechanical-systems (MEMS) cantilever integrated with the top DBR mirror thereby changing the cavity thickness. The use of MEMS tends to limit the tuning speed of the device within the microsecond range. However, the main disadvantage of VCSELs is that they tend to have low output power (i.e., on the order of about hundreds of microwatts or lower). Another disadvantage of traditional VCSELs is their operational wavelengths are limited to short wavelengths of about 850 nm to about 1300 nm.
ECLs basically utilize an external reflector such as a diffracting grating or MEMS mirror to form an external cavity. By mechanically adjusting the external cavity length, the lasing wavelength can be tuned over a wide range. ECLs can also provide high output power and narrow linewidth. However, most of current ECLs are very large, costly, sensitive to environmental changes, and operate with a slow tuning speed on the order of milliseconds. In addition, current ECL designs tend not to be applicable to large-scaled integration.